To start a turbine engine a starter is used which supplies a drive torque to the shaft of the turbine engine. The purpose of such a drive torque is to overcome all the resistive torques exerted by the shaft and which are primarily the result of the aerodynamic drag of the parts which are made to rotate, of the mechanical friction of parts in contact and viscous frictional losses between the parts in contact immersed in the fluid of at least one hydraulic lubrication and/or thermal regulation circuit associated with the turbine engine. The starter gradually accelerates the turbine engine, and when a pre-defined speed of rotation is reached injection and ignition of the fuel in the combustion chamber of the turbine engine are initiated. Then, at another pre-defined speed of rotation, the action of the starter is stopped and the turbine engine continues to accelerate up to its idling speed due to the combustion of the fuel.
Known starters are of the pneumatic type (a turbine fed by compressed air) or of the electric type. Only pneumatic starters are considered in the context of the invention. FIG. 1 shows a pneumatic starter 10 on a fan 11 of a turbine engine of an aircraft. The pneumatic starter 10 is designed to be supplied with compressed air via pipework 12. Generally the air upstream 13 of the pipework 12 is pressurised by means of an auxiliary power unit (APU) located in the aircraft. Sometimes the air is pressurised by a unit on the ground or by air taken from another turbine engine which has already started up. Regardless of the source of the compressed air, air under pressure reaches the upstream side 13 of the pipework as arrow 14 shows, bound for the pneumatic starter 10.
A starter air valve 15 (SAV) regulates the flow of pressurised air in the pipework 12: the valve 15, by opening or closing, is capable of allowing, preventing or limiting the routing of compressed air towards the downstream side 16 of the pipework 12 where the pneumatic starter 10 is located. When closed the starter air valve 15 prevents the pressurised air from reaching the pneumatic starter 10. When the turbine engine starts up, the starter air valve 15 gradually opens, allowing compressed air to pass onto the pneumatic starter 10. The pressurised air then enters the turbine of the starter 10, which transforms the pneumatic energy into mechanical energy to make the shaft of the turbine engine rotate. It should be noted that the airflow reaching the starter 10 is directly linked to the degree of opening of the starter air valve 15.
A significant proportion of unsuccessful start-ups have jamming of the opening or closure of the starter air valve as their source. When jammed closed, the pneumatic starter is never exposed to the pressurised air and the shaft is not made to rotate. When jammed open, it can cause the pneumatic starter to run at excessive speed. Once uncoupled from the shaft the pneumatic starter is not exposed to a resistive torque, but continues to be supplied with compressed air.
Most pneumatic start turbine engines use a butterfly-type starter air valve. In practice a proximity sensor is commonly used to establish the precise moment at which the butterfly completes an opening movement or a closure movement. Depending on its position, the proximity sensor indicates when the valve is fully closed or when the valve is fully opened. It should be noted that such a proximity sensor can be used to tell if the valve is open or if the valve is shut, but is not capable of providing these two items of information for the same valve. Two proximity sensors would be required in order to know when the valve is open and when the valve is closed. Some starter air valves however are not fitted with such sensors, or there may in fact be a fault in the sensor.